Coal is the most abundant and inexpensive energy source on our planet with sufficient reserves to meet a large fraction of the world's energy needs for many decades, even centuries to come. Other energy sources such as wind, solar, geothermal, and nuclear offer limited capacity only. As demand for energy resources increases with accelerating pace to fuel the rapidly growing economies of populated countries like China, India, Brazil and Russia, there is a compelling and impending need to find more efficient and responsible ways to use coal, as these countries both possess vast reserves of this valuable resource and use it in great proportion for their electricity generation. For example, China generates more than 70% of its electricity from coal. That number is about 56% for the US.
Conversion of the chemical energy of coal to electricity ordinarily requires multiple processing steps that suffer from Camot constraints and ultimately results in low conversion efficiencies. Typically, subcritical coal fired power plants operate with poor efficiencies of 33-35%. Recently, emerging coal technologies have slightly improved efficiencies that may reach up to 42-45% for ultra-super critical and integrated gas combined cycle processes. However, these are relatively new and expensive technologies with capital costs in excess of $1700/kW without CO2 capture and more than $2200/kW with CO2 capture [N. Holt, GCEP Advanced Coal Workshop, Provo, Utah, Mar. 15, 16 (2005)].
In contrast, the process of direct electrochemical conversion of coal to electrical energy proposed here is a single step process and is not subject to Camot constraint. Hence, it offers the possibility of achieving substantially higher conversion efficiencies than chemical conversion processes. This is partly due to the high ceiling value of the theoretical efficiency for electrochemical conversion of carbon to carbon dioxide, which remains at about 100% even at elevated temperatures due to near-zero entropy change of the reaction.
Possibility of direct coal conversion in a fluidized bed fuel cell was originally suggested by Gür [T. M. Gür and R. A. Huggins, J. Electrochem. Soc. 139 (#10), L95 (1992), U.S. Pat. No. 5,376,469 Dec. 27, 1994] that utilized a fixed carbon bed in a solid oxide fuel cell. This early attempt employed a thick (1.5 mm wall) stabilized zirconia tube. It was also a preliminary study for proof-of-concept only that was far from being optimized. Nevertheless, it successfully demonstrated that it was possible to draw currents in excess of 30 mA/cm2 and obtain open circuit voltages that agreed with theoretically expected values.
The quest for direct carbon conversion to electricity is not new, however, and has been pursued in waves of activity for over 150 years. The earliest attempt to directly consume coal in a fuel cell was made by Becquerel in 1855[K. R. Williams, in “An Introduction to Fuel Cells”, Elsevier Publishing Company, Amsterdam (1966), Chap.1]. He used a carbon rod as the anode and platinum as the oxygen electrode in a fuel cell that employed molten potassium nitrate as the electrolyte. When oxygen was blown on to the Pt electrode a current was observed in the external circuit. However, his results were not encouraging because of the direct chemical oxidation of carbon by the potassium nitrate electrolyte.
Near the turn of the century, Jacques [W. W. Jacques, Harper's Magazine, 94, 144 (December 1896-May 1897)] used a molten sodium hydroxide electrolyte contained in an iron pot, which served as the air cathode, and a carbon rod as the consumable anode. The cell was operated at about 500° C. and current densities of over 100 ma/cm2 were obtained at about 1 volt. He constructed a 1.5 kW battery consisting of over 100 of these cells and operated it intermittently for over six months. Unfortunately, Jacques did not give reliable information about cell characteristics and life of his battery. Haber and Brunner [F. Haber and L. Bruner, Z. Elektrochem., 10,697 (1904)] suggested that the electrochemical reaction at the anode in the Jacques cell was not the oxidation of carbon but of hydrogen that was produced, along with sodium carbonate, by the reaction of carbon with molten sodium hydroxide. Owing to this undesirable side reaction involving the electrolyte and rendering it unstable in that environment, Baur and co-workers [E. Baur, Z. Elektrochem., 16,300 (1910); E. Baur and H. Ehrenberg, Z. Elektrochem., 18,1002 (1912); E. Baur, W. D. Treadwell and G. Trumpler, Z. Elektrochem., 27,199 (1921)] abandoned the molten alkali electrolytes and replaced them by molten salts such as carbonates, silicates and borates.
In 1937, Baur and Preis [E. Baur and H. Preis, Z. Elektrochem., 43,727 (1937)] suggested that the condition for a chemically stable electrolyte can only be met by the use of an ionically conducting solid electrolyte. For this purpose, they built a battery consisting of eight yttria stabilized zirconia electrolyte crucibles immersed in a common magnetite (i.e., Fe3O4) bath. The anode compartment was filled with coke and the cell was operated at about 1050° C. The open circuit battery potential was 0.83 volts, about 0.2 volts lower than that measured with single cells. At a cell voltage of about 0.65 volts the current density was about 0.3 mA/cm2, too low for practical use. Furthermore, at these high operating temperatures, it is thermodynamically possible to carry out only partial oxidation of carbon, which would hence reduce the efficiency of the fuel cell significantly.
In the last several decades, high temperature fuel cells employing either molten carbonate or solid oxide ceramic electrolytes have been reported. In these cells, coal derived fuels [D. H. Archer and R. L. Zahradnik, Chem. Eng. Progr. Symp. Series, 63,55 (1967)], H2 [J. Weissbart and R. Ruka, in “Fuel Cells”,Vol. 2, G. J. Young (ed.), Reinhold Publishing Corp., New York (1963)] and CH4 [J. Weissbart and R. Ruka, J. Electrochem. Soc., 109,723 (1962)] were employed as consumable gaseous fuels. Presently, the high temperature solid oxide fuel cells under development in various laboratories around the world use H2 derived either from natural gas or from coal.
More recently, there have been several development efforts that utilize some form of a molten medium in an attempt to generate electricity from carbon. The molten media that were employed can be grouped of two categories, namely, molten salts and molten metals, both of which serve to hold the carbon source.
Molten Salt Electrolyte Based DCFC
Scientific Applications and Research Associates, Inc. (SARA) has been involved in developing a molten hydroxide fuel cell operating at 400-500° C. [www.sara.com/energy; “Carbon Air Fuel Cell” U.S. Pat. No. 6,200,697 (Mar. 3, 2001)]. The cell consists of a carbon anode surrounded by a molten hydroxide electrolyte. Air is forced over the metallic cathode where the reduction of oxygen generates hydroxide ions. The hydroxide ions are transported through the molten NaOH electrolyte to the anode where they react with the carbon anode releasing CO2, H2O, and electrons. These electrons travel through the external circuit to the cathode, and generate electricity.
Building upon the earlier work done at SRI International by Weaver and co-workers [R. D. Weaver, S. C. Leach, A. E. Bayce, and L. Nanis, “Direct Electrochemical Generation of Electricity from Coal”, SRI, Menlo Park, Calif. 94025; SAN-0115/105-1 (1979) ] who employed a carbon anode in a molten carbonate electrolyte system for direct conversion of carbon to electricity, Lawrence Livermore National Laboratory [N. J. Cherepy, R. Krueger, K. J. Fiet, A. J. Jankowski, and J. F. Cooper, J. Electrochem. Soc. 152(1), A80 (2005); J. F. Cooper “Direct Conversion of Coal and Coal-Derived Carbon in Fuel Cells”, Second International Conference on Fuel Cell Science, Engineering and Technology, ASME, Rochester, N.Y., Jun. 14-16, 2004; “Fuel Cell Apparatus and Method Thereof”, U.S. Pat. No. 6,815,105 (Nov. 9, 2004) ] has been developing a similar system which employs a molten carbonate electrolyte that holds nanosize carbon particles dispersed in it. The anode and cathode compartments are separated by a porous yttria stabilized zirconia (YSZ) matrix, which serves to hold the molten electrolyte and allows transport of carbonate ions from the anode side to the cathode compartment. Suitable metals such as Ni are employed for anode and cathode materials. At the anode, dispersed carbon particles react with the carbonate ion to form CO2 and electrons, while oxygen from air react with CO2 at the cathode to generate carbonate ions. As the carbonate ions formed at the cathode migrate through the molten electrolyte towards the anode, the electrons liberated at the anode travel through the external circuit towards the cathode generating electricity.
Molten Anode Based DCFC
Yentekakis and co-workers [I. V. Yentekakis, P. G. Debenedetti, and B. Costa, Ind. Eng. Chem. Res. 28, 1414 (1989)] published a paper-study and proposed the concept for and simulated the expected performance of a direct carbon conversion fuel cell employing a molten Fe anode and an yttria stabilized zirconia (YSZ) solid electrolyte immersed in the molten anode. The operating temperature of such a cell would necessarily be higher than the melting point of Fe, which is 1535° C. Indeed, their modeling was done for extremely high temperatures up to 2227° C. (or 2500° K). It was assumed that finely divided carbon particles are dispersed in the molten Fe anode. They suggested coating the cathode side of the YSZ electrolyte with a porous layer of Pt where the oxygen from the air would undergo a reduction reaction. The resulting oxide ions would be transported through the YSZ solid electrolyte towards the anode where they would emerge into the molten Fe bath and react with the dispersed carbon particles. The electrons released during this anodic reaction would travel in the external circuit generating electricity.
A similar approach has been pursued by CellTech Power, Inc., which recently patented [“Carbon-Oxygen Fuel Cell”, U.S. Pat. No. 6,692,861 B2 (Feb. 17, 2004)] a fuel cell that uses a carbon-based anode. Their web site portrays a fuel cell [www.celltechpower.com] that employs molten Sn as anode and reports that the cell operates in a two-step process. During the first phase, the oxygen transported through a stabilized zirconia solid electrolyte oxidizes the molten Sn anode to SnO. In the second step, carbon fuel delivered into the anode compartment reduces the SnO back to metallic Sn, and the cycle is repeated.
The present invention is fundamentally different from these prior approaches. It employs a dense and nonporous solid oxide ceramic electrolyte for selectively transporting oxygen necessary for oxidizing carbon. While others employ either electronically nonconducting molten salt electrolytes or electronically conducting molten metal anodes, the proposed concept uses instead a gas-solid system where mass transport and kinetic rates are significantly higher than for liquid-solid systems. Hence, the expected power densities in this proposal will proportionately be higher. Operationally, it also less complicated to deal with and study reactions in gas-solid interfaces than in the double and triple phase interfaces employed in the molten electrolyte or molten anode gas-solid-liquid systems above.
Properties of Solid Oxide Electrolytes
An important component of the direct coal fuel cell (DCFC) is the solid oxide electrolyte that facilitates selective oxide ion transport and supplies the oxygen for the oxidation of carbon and other reactants (such as hydrogen, sulfur etc) at the anode. Predominantly oxide-ion conducting solids have been known to exist for almost a century [W. Nernst, Z. Elektrochem., 6,41 (1900)]. Among these solids, zirconia-based electrolytes have widely been employed as electrolyte material for solid oxide fuel cells (SOFC).
Zirconium dioxide has three well-defined polymorphs, with monoclinic, tetragonal and cubic structures. The monoclinic phase is stable up to about 1300° C. and then transforms to the tetragonal phase. The cubic phase is stable above 2200° C. with a CaF2 structure. The tetragonal-to-monoclinic phase transition is accompanied by a large molar volume (about 4%), which makes the practical use of pure zirconia impossible for high temperature refractory applications. However, addition of 8-15 m % of alkali or rare earth oxides (e.g., CaO, Y2O3, Sc2O3) stabilizes the high temperature cubic fluorite phase to room temperature and eliminates the undesirable tetragonal-to monoclinic phase transition at around 1300° C. The dopant cations substitute for the zirconium sites in the structure. When divalent or trivalent dopants replace the tetravalent zirconium, a large concentration of oxygen vacancies is generated to preserve the charge neutrality of the crystal. It is these oxygen vacancies that are responsible for the high ionic conductivity exhibited by these solid solutions. These materials also exhibit high activation energy for conduction [T. M. Gür, I. D. Raistrick and R. A. Huggins, Mat. Sci. Engr., 46, 53 (1980); T. M. Gür, I. D. Raistrick and R. A. Huggins, Solid State Ionics, 1, 251 (1980)] that necessitates elevated temperatures in order to provide sufficient ionic transport rates. The electronic contribution to the total conductivity is several orders of magnitude lower than the ionic component at these temperatures. Hence, these materials can be employed as solid electrolytes in high temperature electrochemical cells.
Ionic conduction in these materials is a highly thermally activated process with strong temperature dependence and large activation energy of about 1 eV. In fact, ionic conductivity for the oxide ions increases exponentially with temperature, dictating the need of high operating temperatures for fast transport rates. Therefore, it is desirable for the solid oxide electrolyte to operate between 600 to 1100° C. in order to provide sufficiently fast transport rates for the oxide ions that would make it attractive for practical use.
The chemical potential difference of oxygen across the solid oxide electrolyte is a measure of the open circuit potential given by the Nernst Equation,E=−(RT/nF)ln(PO2′/PO2″)  (1)where E is the equilibrium potential of the fuel cell under open circuit conditions, R is the gas constant, F is Faraday's constant, n is the number of electrons per mole (in the case of O2, n =4), and PO2 denotes the partial pressure of oxygen.